Adding a missing folder to make build work

[ryantxu]

This is a trivial pull request to make an out-of-the box 'build.cmd all' work.

Without the folder, I currently get:

  BUILD FAILED
  C:\workspace\waffle\Source\JNA\build.xml:20: The following error occurred while executing this line:
  C:\workspace\waffle\Source\JNA\build.xml:5: The following error occurred while executing this line:
  C:\workspace\waffle\Source\JNA\build\build.xml:227: The following error occurred while executing this line:
  C:\workspace\waffle\Source\JNA\build\build.xml:93: srcdir "C:\workspace\waffle\Source\JNA\waffle-mock\src-test" does not exist!

(also trying a simple PR to make sure I have things setup correctly)

thanks
ryan

[dblock]

I merged this, thank you.